QuoteThis is not an argue R.G. I stated my opinions accompanied with what I consider as pretty valid arguments. Why persist that there is something wrong with them?I agree - this is not an argument. I have different opinions, and I consider the exploration of them to more than one ply of depth to be a discussion. No argument was intended.
QuoteThat's true in general; however, this forum was started with high consideration for the LM3886 amps, and that continues to be a major consideration. All of the "chip amp" style amps do have ruler-flat response unless you do something to change that, as they are all high-feedback, low output impedance amps unless you go to some lengths to make that not true.
Guitar power amplifiers do not have to have a dead flat frequency response but can be made to react to speaker impedance similarly as tube amplifiers do.
QuoteYou can even make LM3886 exhibit the same character if you want to: It's all about the feedback configuration.This being one way to make that not true.
QuoteSome amplifier models even have active tone control circuits in the feedback loop of the power amplifier.And this being another. Note that this method still leaves a low impedance output with a tight "grip" on the speaker.
QuoteA reason why to make PA circuit's frequency response flat is as valid as the reason why not to. It's all about what the designer wishes to do. If the frequency response of PA is not flat then it's tough luck for limiters that detect only the preamplifier's signal level.That's another version of what I said. We agree there.
QuoteAnyway, a limiter that has a detector hooked to PA output and that attenuates the signal fed to power amp: Is that a preamp-based limiter or power amp-based... Well, we have a semantics issue here and it's quite hard to determine where to draw a line in this. It is easier to make a division between limiters that control the power amplifier's gain or power amplifier's input signal amplitude.I may not have been clear.
You can do the limiting entirely feed-forward by limiting at the preamp output or feed-back by watching the output of the power amp. There are multiple methods for each one.
Power amps are inherently high power opamps these days (neglecting tube amps, since this is a solid state guitar amp forum), and since they are, unless you are forcing them into some nonlinearity, the output follows the input as dictated by the feedback network. Whether you use a low power opamp or a high power opamp to do your limiting makes little difference if you make the assumption that you are not going to let the amp doing the limiting go into clipping - which is the goal you're after in the first place.
That being the case, it is a lower-risk technique to limit before you go into a high power stage, and to do the limiting in a way that whatever you feed the power amp will never force the power amp into clipping under any condition. The down side of doing your limiting in a low power stage ahead of the power amp itself is that (1) you have to use an additional US$0.15 opamp to do it instead of the power amp you would otherwise use (2) you have to do your power limiting at lower voltage, which is more of a design problem than it sounds like, and (3) you may give up a few watts of potential power out in the real power amp by designing not to drive it to the very edge of clipping.
It could be a problem to some people, but I don't consider 1 to be a problem, especially since it removes the risk of getting something wrong in the power amp stage and blowing it. Power amp chips are not as stable as general opamps, and high power oscillation will kill them. The LM3886, for instance, is not stable for gains under five. The TL072 is unity gain stable. You can also put in an RF decoupler and keep any mistakes from the limiting stage from getting into the power amp.
2 is an issue when you are depending on using semiconductor junctions as references for where you do your limiting. The smaller voltages in an opamp limiter as opposed to a power amp limiter make this a bit trickier and require more ability on the part of the designer.
3 is something you've already signed up for by doing soft limiting. The logarithmic response of human hearing make this almost inaudible anyway.
QuoteVaporizing the power amp is a bit far-fetched but I guess it's valid to assume something like that would happen if someone doesn't quite know what he/she is doing.Yep. I can promise you that it can happen. I've done it. I guess I'm one of those people who didn't know what they were doing at one time in the past. Not all of us can always know what they're doing.
QuoteIn my opinion, having the detector at the amplifier output is more accurate way of guessing when PA is approaching clipping levels than determing a certain preamp signal voltage level threshold where the limiter kicks in - at least in cases when power amplifier's gain is dictated by impedance (valid assumption) or when supply sag has to be considered (another valid assumption).It is in my opinion too. But my opinion also is that there are other design reasons for making not having to make this determination accurately. If you don't need to make a razor-sharp determination of where something clips, so much the better.
QuoteIf power amp has dead flat frequency response and doesn't exhibit a notable sag then there is really no need to make things any harder than needed. In practice, many amplifiers work fine even without limiters. ...But they also would work better with them - especially with all kinds of them.I believe that. Of course, I'm after soft-clipping for musical sound, not for protection of the power amp or speakers. IMHO, SS amps should never be let clip. Period. It's too nasty sounding. It's the sound I'm after. I'd like to control how the clipping happens so it's musical. I think this is easier to do in a low power stage than a high power stage. It's not the only way, just the one I like.
QuoteKeeping mind open for alternative methods is always beneficial.I could not agree with you more. I think that keeping an open mind is essential, maybe crucial, to improving one's design abilities.